Summary of Stock Option Transactions Relating to Non-Qualified Stock Option Plans (Detail) - Stock options $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Number of Shares
No. of options, Outstanding at December 31, 2016 | shares	1,327
No. of options, Granted | shares	0
No. of options, Exercised | shares	(313)
No. of options, Cancelled | shares	0
No. of options, Outstanding at December 31, 2017 | shares	1,014
No. of options, Exercisable at December 31, 2017 | shares	1,014
Weighted-Average Exercise Price per Share
Weighted-Average Exercise Price of options, Outstanding at December 31, 2016 | $ / shares	$ 20.53
Weighted-Average Exercise Price of options, Granted | $ / shares	0
Weighted-Average Exercise Price of options, Exercised | $ / shares	22.18
Weighted-Average Exercise Price of options, Cancelled | $ / shares	0
Weighted-Average Exercise Price of options, Outstanding at December 31, 2017 | $ / shares	20.03
Weighted-Average Exercise Price of options, Exercisable at December 31, 2017 | $ / shares	$ 20.03
Weighted Average Remaining Contractual Term (in years)
Outstanding at December 31, 2017	2 years 7 months 6 days
Exercisable at December 31, 2017	2 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding at December 31, 2017 | $	$ 2,669
Exercisable at December 31, 2017 | $	$ 2,669